 It has been noticed in "Arabidopsis" that sometimes when two different plant lines are crossed together, the offspring show signs of hybrid necrosis.This is due to the parent plants containing incompatible NLRs, which when expressed together in the same cell, induce spontaneous HR.This observation raised a hypothesis that plant pathogens can lead to the speciation of plants – if plant populations from the same species develop incompatible NLRs in response to different pathogen effectors, this can lead to hybrid necrosis in the F1 offspring, which substantially reduces the fitness of the offspring and gene flow to subsequent generations.